This invention relates to olefin polymer compositions, for example polypropylene, polyethylene, and poly-1-butene, having improved clarity as a result of incorporating an additive therein, and to additive compositions that improve the clarity of olefin compositions when incorporated therein in modest concentrations.
Low density polyethylene has been known for many years to have a crystalline structure. Additional crystalline polymers such as high-density polyethylene, isotactic polypropylene, poly-1-butene, and poly-4-methylpentene have become available as a result of the development of the so-called stereospecific Ziegler-Natta catalyst system for the polymerization of alpha-olefins.
Such crystalline polymers possess special properties, not available in amorphous polymers, which are of steadily increasing commercial interest and importance. Certain properties of these crystalline polymers, particularly optical and strength properties, and processing characteristics, depend to a very significant extent upon the polymer microstructure.
According to C. J. Kuhre et al (SPE Journal, 1964, Volume 20, pages 1,113-1,119), rapid cooling from the molten state, promotes a favorable microstructure by producing small spherulites and leads to improved clarity and toughness for polypropylene. However quenching is, in general, applicable with maximum effect only to plastic fabricated with relatively thin section thickness, such as film or thin sheet. While some advantage can be gained in terms of improved microstructure by rapid cooling of plastic articles of relatively thick section, the effect is restricted by heat transfer limitations mainly to the surface shell of the plastic. Certain additives promote the formation of smaller, more numerous spherulites in crystalline polymers. Use of such crystallization modifiers has the distinct and important advantage that their effect is not limited to plastic articles of thin cross-section; a more favorable microstructure forms throughout the entire mass of the polymer.
As a crystalline polymer cools from the molten state, crystallites begin to form below the melting point, as supercooling progresses. The crystallites cluster about central nuclei to form spherulites which grow radially outward by addition of more crystallites as crystallization proceeds. Since the spherulites are birefringent their growth can be readily observed with a polarizing microscope equipped with a hot stage.
Use of small amounts of finely-ground, higher-melting polymers, such as polytetrafluoroethylene, nylon, polyhexamethylene terephthalate, or isotactic polyethylene in low-density polyethylene, and nylons 610 or 66 in nylon 11 result in improved transparency, similar to the improvement obtained by rapid quenching of thin films. This improvement is attributed to the smaller, more numerous spherulites promoted by nucleation, in the case of the polymeric additives, and by rapid cooling, which minimizes spherulite size in the case of quenching. Certain inorganic additives, such as very fine silica, also show some nucleating effect in crystalline polymers. There are a number of other substances which are highly effective as nucleating agents in polypropylene.
An example of these new nucleating agents are the metal salts of organic acids. Metals whose carboxylic acid salts are effective include sodium, magnesium, calcium, aluminum and titanium. The organic acid portion of the salt may be chosen from a variety of either mono- or di-carboxylic acids. Examples of suitable monobasic acids are benzoic, cyclohexane carboxylic, diphenyl acetic, and isonicotinic acids. Dicarboxylic acids such as succinic, adipic, sebacic, and phthalic acids give salts of similar activity. The efficienty of the nucleating agents varies with the choice of metal and acid used. Some of the more promising compounds are effective at concentrations well below 0.5% w.
Kuhre et al here summarize the disclosures of P. Wijga in U.S. Pat. Nos. 3,207,735, 3,207,736, and 3,207,738, and M. Wales in U.S. Pat. Nos. 3,207,737, and 3,207,739, all of Sept. 21, 1966. In Wijga (735), the materials which result in the production of polypropylene of improved physical properties are cyclic monocarboxylic acids having the carboxyl group attached to a carbon atom of a 5 to 6 membered ring, and corresponding anhydrides, referred to as "benzoic acid type compounds". The improvement is said to be obtained when a benzoic acid type compound is present as a liquid, dissolved or thoroughly dispersed in the polymer melt prior to the final crystallization thereof by cooling. In Wijga (736) the materials used similarly are aliphatic, cycloaliphatic, and aromatic dicarboxylic or higher polycarboxylic acids and corresponding anhydrides. In Wijga (738) the materials used similarly are terminal aliphatic monocarboxylic acids substituted with carbocyclic groups, also referred to as arylalkanoic acid type compounds. In Wales (737) the materials used similarly are salts of aluminum with certain carboxylic or polycarboxylic acids, and in Wales (739) the materials used similarly are salts of sodium and, to a lesser degree, other metals from Groups Ia and IIa of the Periodic table, with certain carboxylic and polycarboxylic acids.
R. Harrington, in U.S. Pat. No. 3,274,014 of Sept. 20, 1966 disclosed yarn compositions having incorporated a small amount of a metal monoalkyl or monoaryl phosphate, metal dialkyl phosphate, metal alkyl phosphonate, metal alkyl (alkyl phosphonate), or metal dialkyl phosphite that are resistant to ultraviolet light. Nothing is disclosed as to any effect on clarity or crystallization behavior of the yarn polymers, and since the yarns can be pigmented any such effects would have been overlooked.
K. Yamamoto in Japanese Pat. No. 15,185/69 of July 15, 1969 disclosed polypropylene compositions of regulated molecular weight containing organotin compounds as molecular weight modifying agents and esters of acids of phosphorus to prevent discoloration. All the disclosed esters are aliphatic esters, for example dibutyl phosphite, dialuryl phosphite, triethyl phosphate, tributyl phosphate, and bis(2-ethylhexyl phosphate). K. Shirayama in Japanese Pat. No. 12,903/71 of Apr. 2, 1971 disclosed crystalline polypropylene compositions of improved transparency and stiffness containing diphenylphosphinic acid represented by the formula ##STR4## or its magnesium, calcium, sodium, or aluminum salts as a nucleating agent. In Japanese Pat. No. 21,939/71 of June 22, 1971, K. Shirayama disclosed phenylphosphonic acid, phenyl phosphonous acid, phenylarsonic acid, or a Na,Mg,Ca, or Al salt of one of these acids as a nucleating agent for polypropylene. K. Hamada in U.S. Pat. No. 4,016,118 of Apr. 5, 1977 disclosed aliphatic polyolefins with increased transparency and reduced shrinkage containing 0.1 to 0.7% dibenzylidene-sorbitol.